This invention relates to controlling a direct, AC to DC matrix converter to supply controlled DC voltage to a load utilizing precisely controlled, pulse width modulation.
In commonly owned, copending U.S. patent application Ser. No. 09/310,600 filed contemporaneously herewith, a direct, 3-phase AC to DC matrix converter employs switches which are controlled in sequence to directly synthesize a desired average DC voltage waveform at the input terminals of the DC motor, while simultaneously distributing the DC output current among the AC input lines as a sinusoidal waveform in phase with the AC voltage. The difference between the direct DC matrix converter of said copending application and prior DC-PWM converters is that the prior converters create a DC power of a fixed voltage, much the same as a battery, and then utilized some portion of the voltage, as needed, synthesizing a correct DC voltage, on average, by means of pulse width modulation, whereas in said application, the desired voltage at the desired current is synthesized by pulse width modulation directly from the AC mains, while retaining the sinusoidal balance and unity power factor of the AC input currents.
In the system of said application, each switch is turned on and off in each modulation period. As is known, the switching losses in power switches occur only during transition between the non-conducting and conducting states; therefore, reducing the number of commutations will significantly reduce power losses in the switches.
Objects of the invention include providing pulse width modulation synthesis of DC voltage directly from three-phase AC mains with minimal commutation losses, with a minimum of calculations (processor steps), with modulation frequencies as high as 10 KHz or more to provide minimal ripple in the DC voltage and current, with minimal distortion and a unity power factor at the AC mains.
This invention is predicated on my discovery that all switches in a DC matrix converter can be turned on and remain on for two out of three portions of the same or adjacent pulse width modulation periods, when operated in a proper sequence, including two voltage producing portions and one non-voltage producing portion of each pulse width modulation period.
According to this invention, the switch-on time of DC matrix power switches is determined by the ratio of an instantaneous voltage command signal, V*, (indicative of the voltage to be provided by said DC matrix converter) to the instantaneous magnitude, V, of the three-phase AC mains in stationary dq coordinates, along with the phase relationship between the present instantaneous phase of said AC mains voltage in stationary dq coordinates and the leading and lagging boundaries of six phase sectors that span a cycle of said AC mains.
According to the invention further, the phase relationship are expressed in terms of dq quantities, using trigonometric angle-sum relationships and identified by inequalities existing between the voltages of the AC mains expressed in orthogonal dq coordinates and zero.
In still further accord with the invention, the pairs of switches to be used in each portion of a pulse width modulation period are selected by relationships between the components of the AC mains voltage in orthogonal DQ coordinates. The invention may be implemented in DC matrix converters which supply unilateral current, bilateral current, and with or without regeneration.
Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.